Welcome back to ALL THINGS CONSIDERED from NPR News. I'm Guy Raz. Traffic noise and other sounds of city life have taken the romance away for Australia's urban frogs. Air conditioners, jackhammers, generators, they're all drowning out the mating calls of male frogs, and as a result, frog populations are plummeting in the country's urban areas. But there's hope, at least according to a scientist at the University of Melbourne. She says some frogs have figured out a way to work around the human interference in their love lives. From Sydney, Stuart Cohen has the story.

(Soundbite of frogs)

STUART COHEN: That's the sound of a male southern brown tree frog looking for a date. It's music to the ears of a female southern brown tree frog.

(Soundbite of frogs)

(Soundbite of traffic)

COHEN: But add the sounds of nearby traffic, and Dr. Kirsten Parris of the University of Melbourne says the message just isn't getting out.

Dr. KIRSTEN PARRIS (University of Melbourne): The distance over which a male frog can be heard is cut really dramatically by traffic noise from hundreds of meters in some instances, down to maybe only 20 or 50 meters. We're quite concerned that there are frogs out there that aren't getting together because the noise we're making is getting in the way.

COHEN: So some frogs have come up with an interesting strategy for making themselves heard.

Dr. PARRIS: We found that it's changing the pitch of its calls. So going higher up the frequency spectrum, being higher and squeakier, further away from the traffic noise, and this increases the distance over which it can be heard.

COHEN: Here's the old call.

(Soundbite of frog)

COHEN: And here's the new one.

(Soundbite of frog)

COHEN: Now, that might sound like a pretty simple solution, but changing their calls to cope with a noisy environment is actually quite extraordinary for frogs. And while the males have figured out how to make themselves heard above the noise of the city, Dr. Parris says it may not be what the females are looking for.

Dr. PARRIS: When females have a choice between two males calling, they tend to select the one that calls at a lower frequency because, in frogs, the frequency of a call is related to body size. So, the bigger frogs tend to call lower.

(Soundbite of frog)

Dr. PARRIS: And so they also tend to be the older frogs, the guys perhaps with more experience, they know what they're doing, and the women are attracted to those.

COHEN: In other words, the high talkers typically don't get the girls.

Dr. PARRIS: That's right. And if it's very noisy, they can only hear a few of the males that are all calling in a group. So the ones they can choose from the number is reduced. It's like if you're in a noisy cocktail bar, for example, and there are men everywhere, you can only see and hear the three that are closest to you. You either choose to go with one of them, or you spend more energy going to search the room to find someone who looks a little bit more promising. And for females, the longer the time they spend hopping around looking for mates, the more time they're exposed to predators and the more energy they use. So there's a cost to spending a lot of time searching for mates.

COHEN: Frog populations in Melbourne have dropped considerably in the seven years since Dr. Parris began her research. It's not just noise that's been the culprit. Climate change has brought a decade of drought to parts of Australia. Since frogs often serve as the environmental canary in a coal mine, Dr. Parris says their demise is worth paying attention to.

Dr. PARRIS: Urban habitats are expanding very quickly. At the moment, they only cover about four percent of the Earth's terrestrial surface. And I think the fact that frogs and other animals that communicate acoustically, such as birds, are struggling in urban habitats shows us that the urban habitat is not a healthy place for people, either. Noisy places tend to lead to higher stress levels amongst people, difficulty sleeping, and the quality of life is reduced.

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